On Tue, Nov 20, 2018 at 01:53:47PM +0100, Dmitry Vyukov wrote:

On Tue, Nov 20, 2018 at 1:05 PM, Greg Kroah-Hartman
[off-list ref] wrote:

quoted

On Tue, Nov 20, 2018 at 12:34:40PM +0100, Dmitry Vyukov wrote:

quoted

On Thu, Nov 15, 2018 at 1:56 AM, Greg Kroah-Hartman
[off-list ref] wrote:

quoted

On Thu, Nov 15, 2018 at 08:36:17AM +0800, Ming Lei wrote:

quoted

quoted

So even if you think the kernel is not going to do this, remember, you
have no control over it.  Reference counted objects are done this way
for a reason, you really do not know who has a reference and you really
do not care.

You are just papering over the real issue here, see my previous email
for how to start working on resolving it.

IMO, there isn't real issue, and the issue is actually in 'delay release'.

Nope, sorry, that is not true.

quoted

Please look at the code in block/blk-mq-sysfs.c, both q->mq_kobj and all
ctx->kobj share same lifetime with q->kobj, we even don't call get/put
on q->mq_kobj & all ctx->kobj, and all are simply released in q->kobj's
release handler.

How do you "know" you are keeping those lifetimes in sync?  The joy of a
kobject is that _ANYTHING_ can grab a reference to your object without
you knowing about it.  That includes userspace programs.  Yes, sysfs is
now much better and it trys to release that reference "quickly" when it
determines you are trying to delete a kobject, but it's not perfict,
there are still races there.

And that is what the delay release code is showing you.  It is showing
you that you "think" your reference counting is wrong, but it is not.
It is showing you that if someone else grabs a reference, you are not
correctly cleaning up for yourself.

Never think that you really know the lifetime of a kobject, once you
realize that your code gets simpler and you can then just "trust" that
the kernel will do the right thing no matter what.

Because really, you are using a kobject because you want that correct
reference counting logic.  By ignoring that logic, you are ignoring the
reason to be using that object at all.  If you don't need reference
counting, then don't use it at all.

And if you need sysfs files, then you need to use the kobject and then
you need to handle it properly, because again, you do NOT have full
control over the lifetime of your object.  That's the basis for
reference counting in the firstplace.

So this code is broken without me evening having to look at it, please
fix it to handle release properly.  Again, the kernel tried to tell you
this, but you hacked around the kernel core to remove that warning
incorrectly.  Please go read the kobject documentation again for even
more details about this than what I said here.

thanks,

greg k-h

Whoever is the right person to fix this, please prioritize this to the
degree possible.
This issue does not allow to use DEBUG_KOBJECT_RELEASE in any
automated testing (in particular syzbot) on both upstream and stable
trees. We have to disable it for now, so other bugs won't be noticed
and will pile up.

Patches for this have already been posted :)

This is great.
What is the patch name? I can't find anything that looks relevant on
LKML searching by kobject.

https://marc.info/?l=linux-block&m=154270006101625&w=2

Thanks,
Ming